Under OS X, Safari does not accept to launch it. Uncheck the "Open "safe" files after downloading" box in Safari preferences, then the jnlp files get downloaded to the Downloads folder. After that, you must show and open it from the finder instead of Safari.

Under Linux you should be more lucky in using Firefox.

After that you must decrease the security level to Medium (the lowest value) in the Java Control Panel

Under Windows the complete Java Controle Panel to search is javacpl.exe

A security warning screen appears, just check 'I accept the risk and I wish to run the application' and click Run.

If you still have a problem to launch the jniz.jnlp file go the the Java Control Panel. Add the following url: http://www.jniz.org in the exception site list as explained here or here (security tab, edit site list).

Under Linux, the Java Control Panel is a little bit more complicated to find. With an installed OpenJdk for instance, you can use the IcedTea plugins located in /usr/share/applications. With the Oracle version of Java run the jcontrol or ControlPanel command in a terminal.

LilyPond

Unfortunately it seems that Linux or OS X do not accept to launch LilyPond (Midi, LilyPond, PDF export) from the jnlp file. The only alternative is to install Jniz on your local disk.

Under Windows If you want to export LilyPond, Midi, PDF files from this web app please install Lilypond before.

if you get an error message type the Lilypond folder path in the Lilypond and Python fields (Settings menu, LilyPond export options). It should be: